Method for recovering chemicals using recovery boiler having at least 2 different melt sections

ABSTRACT

Chemicals are recovered during the production of cellulose pulp by forming two different sulfidity melts in two different sections of a soda recovery boiler, and then producing cooking liquors from different melts in dissolving tanks. The liquors may be causticized and/or oxidized, or may be utilized uncausticized. Uncausticized green liquor having a sulfidity of about 70-90% may be used early in a kraft cooking cycle, followed by treatment with low sulfidity oxidized and causticized white liquor. In the continuous digestion of cellulosic pulp, oxygen may be added to the recirculation loops below the black liquor withdrawal to enhance the alkalinity of the slurry and to assist in delignification. Oxidized cooking liquor with low sulfidity can be added to oxygen bleaching and alkali extraction stages downstream of the digester, and a portion of the high sulfidity cooking liquor can be used to produce acid used in an ozone bleaching stage.

BACKGROUND AND SUMMARY OF THE INVENTION

As disclosed in pending application Ser. No. 7/788,151, filed Nov. 5,1991 now abandoned there are circumstances in which it is desirable toproduce two different cooking liquors having different sulfidity in theproduction of cellulosic pulp, particularly in the production of kraftor sulfite paper pulp. The invention relates to a particular manner ofeffecting production of the two different cooking liquors, by formingtwo different melts in a soda recovery boiler. Utilizing this basicconcept, it has been found that a wide diversity of pulp treatmentprocedures may then be employed, utilizing a wide variety of differenttypes of cooking liquors. For example, it has been found that when it ispractical to make different cooking liquors from different melts, undersome circumstances it is not necessary to causticize the melts afterthey are dissolved, but rather they have a high enough sulfidity to usethe liquor produced (e.g. green liquor) directly in the early stages ofpulp cooking. Also, it has been found according to the invention that byoxidizing various pulp treating liquids, the alkali content thereof canbe increased, which is useful both at the end of the digestion process,and in some subsequent treatment stages such as oxygen bleaching andalkali extraction. Alternatively, or in addition, it has been found thatusing a high sulfidity liquor, acid can be manufactured which is used insubsequent acidic treatments of the pulp, such as ozone bleaching.

The methods according to the invention are facilitated by utilizing apulp mill having a recovery boiler with two different melt producingvolumes. The conditions within the volumes may be kept separate; forexample, one volume may have oxidizing conditions and the other reducingconditions. Also, the amount of sulfur in the waste gases that aredischarged can be reduced by providing the low sulfidity melt producingvolume above the high sulfidity melt producing volume so that sodiumcontaining off gases from the low sulfidity melt producing volume willreact with sulfur and the off gases from the high sulfidity meltproducing volume to produce compounds that are more easily removed fromthe waste gas stream.

According to one aspect of the present invention, a method of recoveringchemicals during the production of cellulose pulp usingsulfur-containing chemicals is provided. The method comprises thefollowing steps: (a) Providing a first fluid waste stream ofsulfur-containing chemicals from the production of cellulose pulp,having a first sulfur content. (b) Providing a second fluid waste streamof sulfur-containing chemicals from the production of cellulose pulphaving a second sulfur content, greater than the first sulfur content.(c) Combusting the first and second fluid waste streams separately toproduce first and second melts, the second melt having the second sulfurcontent, and the first melt the first sulfur content. And (d) dissolvingthe first and second melts to produce first and second liquors, thefirst liquor having the first sulfur content, and the second liquorhaving the second sulfur content. The first and second fluid wastestreams are preferably formed by heating a waste liquor (e.g. blackliquor) to drive off sulfur-containing gases (such as DMS), recoveringthe sulfur from the sulfur-containing gases, splitting the heated wasteliquor into the first and second different waste gas streams prior tostep (c), and prior to or coincident with step (c) adding the recoveredsulfur to only the second of the split streams.

Steps (a)-(d) are preferably practiced to produce a first liquor havinga sulfidity of about 30% or less, and a second liquor having a sulfidityof about 40% or more, typically about 60-90%, and--where the secondliquor is to be used uncausticized in early stages of kraft cooking--asulfidty of about 70-90%. There is also preferably the further step (e)of adding other sulfur-containing constituents from the production ofcellulose pulp to the second split stream prior to or coincident withthe practice of step (c) for that stream. The constituents added to thesecond split stream may contain one or more of the following: flyashfrom the soda recovery boiler, waste acid from a tall-oil plant, wasteacid from a chlorine dioxide plant, and sulfurous discharge gas from apulp digester.

The first liquor may be treated with oxygen to increase the hydroxideion concentration thereof, and then practicing a further step of,without intervening causticization treating the cellulose pulp in laterstages of cook with the hydroxide-ion concentration enhanced firstliquid. The first and second liquids may or may not be causticizeddepending upon the particular treatment sequences and the uses to whichthey will be put.

The invention also comprises a method of continuously producingcellulose pulp utilizing a continuous digester and first and secondsulfur-containing cooking liquors, comprising the following steps: (a)Feeding comminuted cellulose material slurry entrained in the secondsulfur-containing cooking liquor into the top of the continuousdigester, the slurry moving downwardly in the digester during treatment.(b) Withdrawing black liquor from a first portion of the digester. (c)Below the first portion of the digester, withdrawing treatment liquorand recirculating it, and adding the first sulfur-containing cookingliquor to the withdrawn treatment liquor prior to recirculation. And (d)adding oxygen to the recirculated liquor. Step (d) is practiced tosignificantly raise the hydroxide ion concentration and thus thealkalinity of the slurry, and to assist in delignification. Typicallythe first cooking liquor has a lower sulfur content than the secondcooking liquor (e.g. about 30% or below compared to about 50% or above).The black liquor withdrawn in step (b) is acted upon as by heatingpursuant to the procedure disclosed in U.S Pat. No. 4,929,307 (thedisclosure of which is hereby incorporated by reference herein) toproduce two different fluid waste streams, which are used to produce twodifferent melts as described above.

The invention also comprises a method of producing cellulose pulp from aslurry of comminuted cellulose material by the following steps: (a)Digesting the slurry by subjecting it to cooking with sulfur-containingcooking liquor at digesting conditions, black liquor being produced inthe process. (b) Withdrawing black liquor produced during step (a). (c)Treating the pulp after step (a) in several alkali or acid treatmentstages. (d) Producing cooking liquor from the black liquor withdrawn instep (b). (e) Oxidizing some of the cooking liquor from step (d) toincrease the hydroxide ion concentration thereof. And (f) using theoxidized cooking liquor from step (e) to increase the alkali contentduring the practice of at least one of the alkali treatment stages ofstep (c). Step (c) includes oxygen bleaching and alkali extractionstages, and step (f) is practiced to add oxidized waste liquor to theoxygen bleaching and alkali extraction stages. At least a portion of theoxidized white liquor is recovered from step (f) utilizingcountercurrent washing, and it is utilized in step (a). There may alsobe the step of utilizing a portion of the second melt or the secondcooking liquor to produce acid, and then utilizing the acid in at leastone acid treatment stage (e.g an ozone bleaching stage) during thepractice of step (c). However, the acid can also be produced fromsulfurous gases generated in the heat treatment of the black liquor.

According to still another aspect of the present invention, a method ofkraft digesting comminuted cellulosic fibrous material is provided. Themethod comprises the steps of: (a) First treating the cellulosicmaterial, in slurry form, with uncausticized green liquor having asulfidity of about 70-90% at kraft digestion conditions, and then (b)treating the slurry with a second sulfur-containing liquor having asulfidity of about 30% or less. Step (b) may be practiced with acausticized, oxidized, white liquid.

The invention also relates to a pulp mill and a soda recovery boiler.The pulp mill comprises: (a) Means for providing a first fluid wastestream of sulfur-containing chemicals from the production of cellulosepulp, having a first sulfur content. (b) Means for providing a secondfluid waste stream of sulfur-containing chemicals from the production ofcellulose pulp having a second sulfur content, greater than the firstsulfur content. (c) Means for combusting the first and second fluidwaste streams separately to produce first and second melts, the secondmelt having the second sulfur content, and the first melt the firstsulfur content. And (d) means for dissolving the first and second meltsto produce first and second liquors, the first liquor having the firstsulfur content, and the second liquor having the second sulfur content.The means (c) preferably comprises a soda recovery boiler having atleast two different melt sections and a common waste gas discharge, thefirst melt being produced in a first melt section, and the second meltin a second melt section. The first melt section may be vertically aboveand spaced from the second melt section, the first section discharginggases into the common waste gas discharge substantially above thedischarge of gases from the second melt section into the common wastegas discharge. The first and second liquors may be separately clarifiedand causticized to produce a common supply of lime mud, and the lime mudmay be fed to a lime reburning kiln to be calcinated.

The soda recovery boiler according to the invention comprises: Acombustion chamber. Partition means for dividing the combustion chamberinto first and second melt producing volumes. Means for adding a firstfluid stream to only the first melt producing volume. Means for adding asecond fluid stream to only the second melt producing volume. Means forwithdrawing a first melt from only the first melt producing volume.Means for withdrawing a second melt from only the second melt producingvolume. And a common waste gas discharge from the combustion chamber.The boiler according to the invention can have the modificationsdescribed above depending on the type of the pulp mill.

According to the present invention it is possible to produce pulp havinga low kappa number, and good strength, with a high yield of pulp, and/orto minimize sulfur emissions from the mill. In view of the enhancedqualities of pulp produced according to the invention it is possible toeliminate chlorine and chlorine dioxide as bleaching chemicals, thusfurther minimizing any adverse environmental impact from the pulp mill.

The invention is particularly suited for combination with extendeddelignification kraft pulping techniques, such as the Kamyr, Inc.modified continuous cooking and extended modified continuous cookingtechniques. In such techniques, about 60-80% of the total amount ofwhite liquor is added at the beginning of the cook, while the rest isadded at the end of the cook. By utilizing different sulfidity cookingliquors in the practice of these extended delignification kraft cooks,the results can be enhanced.

In sulphate cooking wood is treated with white liquor containing NaOHand Na₂ S, so that lignin is dissolved and the cellulose fibers arereleased. The mixture of cellulose fibers (pulp) and cooking chemicalsis treated with waster so that black liquor is generated. The blackliquor is concentrated by evaporation. The concentrated black liquor iscombusted in a soda recovery boiler and the chemical melt thus formedand mainly containing Na₂ S and Na₂ CO3 is dissolved into water, wherebygreen liquor is formed. The green liquor is then causticized withcaustic lime (CaO) to white liquor containing NaOH. Another product ofthe causticization reaction is lime mud primarily being formed fromCaCO₃. The white liquor is supplied to a digester house and the lime mudis calcinated in a lime sludge reburning kiln to be reused as causticlime during the caustization.

The pulp discharged from the digester may be, if desired, bleached. Inorder to decrease harmful environmental effects the cellulose pulpingindustry is striving to replace conventional chlorine bleaching withother alternatives, for example, by use of ozone. In ozone bleaching thepulp exiting the cooking apparatus is prebleached with oxygen underalkali conditions. Thereafter, the pulp is bleached with ozone, which issucceeded by alkali extraction. The alkali used in the oxygen bleachingand alkali extraction stage is oxidized white liquor. After eachtreatment the pulp is washed to remove the chemicals from the pulp, forexample, by countercurrent washing. The washing agent from the lastwashing stage, to which the chemicals have been concentrated, is led tothe recovery process of the chemicals.

The various detailed features of the invention will become clear from aninspection of the detailed description of the invention and from theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow sheet illustrating cooking and subsequenttreatment of cellulose pulp employing some of the techniques accordingto the present invention;

FIG. 2 is a schematic illustration of an exemplary embodiment ofapparatus for manufacture of two different sulfidity cooking liquorsfrom two different melts;

FIGS. 3 and 4 illustrate alternative configurations of equipment foraccomplishing the same basic objective as the equipment in FIG. 2;

FIG. 5 schematically illustrates a recovery boiler used in themanufacture of oxidized white liquor;

FIG. 6 schematically illustrates an alternative boiler construction of asoda recovery boiler in accordance with the present invention;

FIG. 7 is a schematic diagram illustrating one alternative method ofproducing different sulfidity liquors according to the invention;

FIG. 8 is a schematic representation of apparatus utilized to practicevarious method aspects of the invention, particularly during extendedmodified continuous cooking of kraft or sulfite pulp; and

FIG. 9 is a schematic diagram like that of FIG. 7 only illustrating analternative procedure.

DETAILED DESCRIPTION OF THE DRAWINGS

According to FIG. 1, wood chips 1, as well as cooking liquor, areintroduced to a digester 1'. In accordance with the present inventionhigh sulfidity white liquor 2 is supplied to the beginning of the cook,and during the cook low sulfidity liquor 3 is gradually added thereto.Alternatively, non-causticized green liquor high in sulfur may be addedto the beginning of the cooking. The sulfidity of the high sulfiditycooking liquor is preferably above 40% and that of the low sulfidityabout 30% or less. When using uncausticized high sulfidity cookingliquor, the sulfidity is preferably above 60%, typically about 70-90%.By so cooking, a low kappa number is achieved as well as a good pulpquality. At the end of the cook the pulp is separated from the wasteliquor by washing, for example, with a countercurrent washing agent 4cfrom the bleaching plant. The waste liquor and the waste water 5 are ledto the chemical recovery processes. The unbleached pulp 6 is supplied tothe bleaching plant.

The pulp 6 is typically first prebleached with oxygen under alkaliconditions at 4'. The alkali used is preferably low sulfidity whiteliquor 3a, which is oxidized to deactivate sulfides at 9. Subsequent tothe oxygen bleaching at 4', the pulp is washed again using washing agent4b flowing countercurrently from the next stage. The washed pulp 7 isthereafter bleached with ozone under acidic conditions in 4".

The pH of the ozone bleaching stage is decreased either by the wasteacid from a chlorine dioxide plant--operating, for example, withMathieson, R8 and R9 processes--or using acid obtained therefrom, orusing acid obtained from cooking chemical circulation. In the lattercase, the acid can be produced, for example, from sulfurous gas, whichis generated in the heat treatment of black liquor or the combustion ofodorous gases. An alternative acid source is also the waste acid from atall-oil digester house.

Subsequent to the ozone bleaching at 4", the pulp is washed, as in theprevious stages, with a washing agent 4a from a subsequent stage,whereby a portion of the acid used for the adjustment of pH is returnedto the chemical circulation. Thereafter, the pulp 8 is led to alkaliextraction at 4'". The pH is adjusted, as in oxygen bleaching, with alow sulfidity, oxidized white liquor 3a. A portion of the chemicals inthe white liquor is returned to the chemical circulation when the pulpis finally washed in countercurrent washing.

In the prior art it is possible to achieve a kappa number of about 10with the present pulp manufacturing processes, which comprise cooking,oxygen bleaching and ozone bleaching, whereby the pulp must yet bebleached with chlorine. By utilizing a method in accordance with thepresent invention it is possible to achieve a kappa number ofapproximately 2, so that the use of chlorine may be avoided.

In the art, the black liquor formed during cooking is concentrated, andcombusted in a soda recovery boiler for generating a melt containingsodium carbonate and sodium sulfide. The melt is dissolved into water toform green liquor. The green liquor typically is reacted with CaO (i.eis causticized) to produce white liquor containing NaOH and Na₂ S.

The viscosity of black liquor may be irreversibly decreased and thusevaporability thereof may be improved by the heat treatment of blackliquor at a temperature higher than the cooking temperature (as in U.S.Pat. No. 4,929,307). Sulfurous gases are released from black liquorduring heat treatment, and thus by adjusting the temperature and/or theretention time, it is possible to affect the sulfur content of blackliquor, as in Finnish published application 85515 and copending U.S.application Ser. No. 07/840,080 (pending) filed Feb. 21, 1992, andcopending U.S. application Ser. No. 07/788,151 filed Nov. 5, 1991 nowabandoned.

The cooking liquors of different sulfur contents required in a pulp millin accordance with the present invention are manufactured from meltshaving different sulfur contents. Such melts are possible to produce byusing a soda recovery boiler RB in accordance with the invention, thelower part of which is divided by a partition wall into two (or more)sections, as illustrated in FIG. 2. The concentrated black liquor isdistributed into both sections. The wash (fly ash) from the electricfilters of a soda recovery boiler and also other sulfur sources of apulp mill, such as sulfurous gases from a digester house, are led to themelt in the combustion space of a second section of RB at the bottom, toa bed on the melt, or to a gas space by means of supply means associatedwith the boiler RB. Accordingly another melt is formed on the otherside. The melts are transferred to a white liquor production plant inaccordance with the present invention, in which they are dissolvedseparately in individual dissolving tanks, respectively. The greenliquors generated thereby are each brought through individual greenliquor clarifiers to individual causticizing tanks 18, 19, respectively.Two different cooking liquors 18', 19' are thus generated, one of whichis of high sulfide content and the other of low sulfide content. Thehighly sulfurous green liquor 19' may remain non-causticized, especiallyif the pH value of the cooking is desired to be maintained low or if theNa₂ CO ₃ content of the green liquor is low.

If the cooking liquors 18', 19' are (separately) causticized, the limemuds produced may be combined so that washing of the lime mud and thecombustion of the lime sludge in reburning kiln may be carried out inone stage. Alternatively, the lime muds may be washed separately and bejoined just before combustion.

FIG. 2 illustrates an exemplary embodiment in accordance with thepresent invention, in which sulfurous compounds may be distributed todifferent sections of a soda recovery boiler RB for the manufacture ofdifferent melts. The concentrated black liquor 10 is brought from theevaporation plant and heat treated by pressure heating at 11 (e.g.according to USP 4,929,307), whereby sulfurous gases 13 are formed andin which gases, for example, 40% of the sulfur of the black liquor isentrained. Black liquor 12, the sulfur content of which is thusdecreased, is evenly distributed between sections I and II at the bottomof the soda recovery boiler RB. Also, other sulfur sources of the pulpmill in addition to the sulfur gas 13 generated in the pressure heating,such as fly ash 15 and sulfurous gases 14, are passed to the section I,the section I producing a high sulfidity melt 17. Melt 17 contains about70% of the sulfur in the black liquor 10 and also some sulfur from otherpossible sulfur sources. The low sulfidity melt 16 thus includes about30% of the total sulfur amount of the black liquor.

Black liquor may, of course, be divided at any desired proportionsbetween the different portions of the soda recovery boiler. Thus, it isalso possible to influence the sulfur distribution between differentportions. A high sulfidity cooking liquor can thus have sulfidities, forexample, from 40% to 90%, and a low sulfidity one can have sulfiditiesfrom 10% to 30%.

After the melts 16, 17 are formed, they are directed to dissolving tanks18, 19, respectively, to produce different sulfur content liquors 18',19', respectively.

The heat treatment of black liquor is not an absolute necessity to bringabout the sulfidity differences. If, for example, only the ash in thesoda recovery boiler, which includes about 30% of the total sulfurcontent and 15% of the sodium, is passed to one section (I) and blackliquor is evenly distributed to different sections (I, II), 65% of thetotal amount of sulfur and 55% of the total amount of sodium is obtainedon the highly sulfurous side. While the original sulfidity of whiteliquor was 35%, the white liquors thus obtained would have sulfiditiesof about 40 and 30%.

FIG. 3 illustrates another exemplary embodiment for the manufacture ofwhite liquors having different sulfur contents. The pre-evaporator blackliquor 20 is heat treated in a reactor 21 to decrease the viscosity ofthe liquor for the final evaporation. The black liquor concentrated inthe evaporator 22 is divided into two so that one portion 23 is leddirectly to the section I at the bottom of the soda recovery boiler RB.The other portion 24 is heated treated in vessel 28 to further decreasethe sulfur content thereof. The black liquor 25 thus treated is broughtto section II, where it is combusted to low sulfidity melt. The ash 26of the soda recovery boiler RB and the sulfurous gases 27 from both ofthe heat treatment reactors 21, 28 are added to the section I of theboiler RB. In this case, different conditions may be used in differentheat treatment stages, and thus control the sulfur distribution.

FIG. 4 illustrates an alternative construction to the embodiment inaccordance with FIG. 3. The pre-evaporated black liquor 30 is dividedinto two. A portion 31 is supplied to the heat treatment reactor 32, inwhich liquor is heated to decrease the viscosity. Then the liquor 33 isconcentrated in the evaporator 34 and guided to the bottom section I ofthe soda recovery boiler RB. The other portion 35 of the pre-evaporatedblack liquor is guided to a more intensive heat treatment in the reactor36 to decrease the viscosity and to achieve an effective sulfurseparation. The low sulfidity black liquor 37 thus obtained isevaporated in an evaporator 38 and passed to section II at the bottom ofthe soda recovery boiler RB in order to obtain the low sulfidity melt41. The sulfurous gases 39 generated in both heat treatments, ash 40,and other possible sulfur sources are guided to the section I of thesoda recovery boiler in order to obtain high sulfidity melt 42. It is,of course, also possible to separate sulfurous gases by other means likegasification, pyrolysis or the like, to create sulfidity differences.

FIG. 5 illustrates yet another embodiment in accordance with the presentinvention, according to which it is possible to produce oxidized whiteliquor, which is required in the bleaching plant. The bottom of the sodarecovery boiler RB is divided into several sections I, II and III.Sections II and III contain low sulfidity melt and section I highsulfidity melt. By maintaining oxidizing conditions in section IIIcontaining low sulfidity melt, it is possible to get oxidized lowsulfidity melt 50 containing Na₂ SO₄. The two sections I and II havereducing conditions, whereby high Na₂ S-containing melt 52 and low Na₂S-containing melt 51 are formed. Thus, it is possible to carry out theoxidization of white liquor and thus eliminate one of the odor sourcesin a pulp mill.

FIG. 6 illustrates an alternative partition means for dividing thebottom of a soda recovery boiler into sections I and II for themanufacture of melts having different sulfur contents. By arrangingsection I containing high sulfidity melt lower than section IIcontaining low sulfidity melt, the discharge gases of the high sulfuricsection rise into contact with the gases of the low sulfuric sectioncontaining sodium. Thus, the sulfurous compounds react with sodiumvapors and are recovered in conventional electric filters (electrostaticprecipitators). Consequently, it is possible to decrease the sulfuremissions of a soda recovery boiler RB. Alternatively, it is possible toarrange a high sulfidity section in the middle of a soda recoveryboiler, whereby a low sulfidity portion surrounds it. Thus, thedischarge gases of the high sulfidity portion are divided evenly in thegases of the low sulfurous portion.

A soda recovery boiler plant in accordance with the present inventionmay comprise also at least two individual boilers. According to oneembodiment the boilers share a heat recovery system, and in addition tothat they may share a flue gas cleaning system. Alternatively, theboilers may be entirely separate. The ash recovered from the common fluegas cleaning system is returned to one of the boilers, whereby thehighly sulfurous melt is naturally obtained in said boiler. If theboilers are entirely separate, the ashes recovered therefrom arereturned only to one of the boilers to form the highly sulfurous melt.

In order to control the corrosion problems at least a portion of thesoda recovery boiler RB, which is in contact with the high sulfuricmelt, is preferably coated. An appropriate coating is a refractorylining, which may extend also to the area, which contains a lot ofsulfur gases, e.g., see lining 57 in FIG. 6. The refractory lining 57may extend more than 10 m above the melt surface.

FIG. 7 schematically illustrates a modified method according to thepresent invention in which black liquor 60 from the digester is fed tothe heat treatment apparatus 61 (as in U.S. Pat. No. 4,929,307), andtreated, e.g. at a temperature of about 200° C., for an extended periodof time. Sulfur-containing gases such as dimethyl sulfide,methylmercaptan and hydrogen sulfide pass into line 62, and the moreconcentrated black liquor, but with a lower sulfur content, in line 63to recovery boiler 64. In the heat treatment stage 61 over 50% of thesulfur may be separated so that the sulfidity of the black liquor inline 63 is about half of its normal sulfidity of about 30-40%. Bycontrolling the heat treatment conditions, the amount of sulfur can becontrolled.

After combustion of the black liquor in the recovery boiler 64, a melt65 is obtained. A portion of the melt passes in line 66 while anotherportion passes in line 67. Line 66 is connected to a conventionaldissolving tank 68 which produces green liquor in line 69. Preferably,the green liquor in line 69 is subjected to an oxygen treatment in whichoxygen is brought into contact with the green liquor to enhance thehydroxide ion concentration (alkalinity). Oxygen added to the oxygentreatment apparatus 70 may be excess oxygen from an ozone bleachingstage. The conversion of the sulfur component of the green liquor inline 69 is as follows: 4 S²⁻ +O₂ +2 H₂ O =2 S ₂ ²⁻ +4 OH⁻ and othercorresponding reactions, providing an increase in OH⁻ concentration ofthe liquor (71). This makes the liquor more suitable for a cookingliquor at the end of the cook. By treating the cooking liquor in thisway, the need to causticize it is greatly reduced. The oxygen treatedgreen liquor from 71 is used at the end of the cooking process withoutcausticizing it, or fed to stages 4' and 4'" (see FIG. 1).Alternatively, as indicated in dotted line at 72 in FIG. 7, the lowsulfidity cooking liquor may bypass the oxygen treatment and be useddirectly in the later stages of the cook, for oxygen or alkali stages,or the like.

The second portion of the melt from recovery boiler 64 in line 67 is notdissolved as such, but rather it is combined with recovered sulfur.Recovered sulfur is obtained by passing the sulfur-containing gases inline 62 to a converter 73 (utilizing apparatus such as described incopending application Ser. No. 07/840,080, filed Feb. 21, 1992 (pending)or Ser. No. 07/788,151, filed Nov. 5, 1991 now abandoned). Thesulfur-containing compounds in line 74 are combined with the melt fromline 67 to produce a very high sulfidity melt 75, which then passes intoa dissolving tank 76 to produce a liquor in line 77 that has a sulfidityof over 50%, and preferably about 60-90%. The liquid in line 77 is addedto the beginning of the cook. It is possible to use the liquor in line77 directly for cooking, without causticizing it, if it has a sulfiditylevel above about 70%.

FIG. 8 schematically illustrates apparatus for digesting comminutedcellulosic fibrous material to produce paper pulp, particularly in thekraft and sulfite processes in which cooking liquor containing sulfurcompounds is utilized. The comminuted cellulose material is fed in line80, ultimately to the digester but preferably through an impregnationvessel 81 in which high sulfidity cooking (e.g. white or green) liquor,e.g. from line 77 of FIG. 7, is added. The cellulose entrained in highsulfidity cooking liquor passes in conduit 82 to the top of the digester83. Black liquor is withdrawn in line 84 through screens 85, and is usedas the feed liquor for the lines 10, 20, 30 or 60 in FIGS. 2 through 4and 7 respectively. Lower in the continuous digester 83, below the blackliquor screens 85, are two circulation loops 86, 87, the loop 87 being awash circulation loop. Spent treatment liquid is withdrawn in line 88 inloop 86 to which low sulfur content white liquor (e.g. from 71, 72) isadded, and then is returned in line 89 to the digester 83. The additionof white liquor in a circulation loop 86 is known per se, utilized inthe Kamyr, Inc. MCC™ process. However, according to the invention, thecooking liquor added in line 88 is low sulfidity cooking liquor.

Also according to the invention oxygen can be added to the liquor inline 89. The oxygen from source 90 added in line 89 converts sulfur ionsto obtain hydroxide ions to raise the alkalinity, and ultimately helpsdelignify the pulp at the end of the cook.

In the circulation loop 87, spent liquid is withdrawn in line 91, andagain low sulfidity cooking liquor from lines 71 or 72 is added. Theaddition of white liquor to a wash circulation loop 87 is known per sebut in the Kamyr, Inc. EMCC™ process. The liquor returned in line 92 canbe made more alkaline by the addition of oxygen from source 93, as withthe first circulation loop 86. The final paper pulp produced isdischarged in line 94, and then is led to further stages (such as theoxygen, ozone, and alkali stages 4' 4" and 41'" illustrated in FIG. 1).

FIG. 9 schematically illustrates a modified form of the system of FIG.7. In its modified form, structures having the same function as those ofthe FIG. 7 embodiment are illustrated by the same reference numeral onlypreceded by a "1".

In the embodiment of FIG. 9, the cooking liquor from dissolving tank 168in line 169, passes through a conventional clarifier 95 and then to acausticizer 96, to produce the final low sulfidity white liquor in line171. Similarly, the high sulfidity liquor in dissolving tank 176 passesto clarifier 97 and causticizer 98 to produce a high sulfidity whiteliquor 177. The lime mud separated from the causticizers 96, 97 passesinto common line 99, and then is washed in wash stage 99' and thenpassed to a lime reburning kiln or the like.

It will thus be seen that according to the present invention methods andapparatus have been provided which greatly enhance the ability toproduce cellulose pulp using sulfur-containing cooking chemical in anefficient, and effective manner. According to the invention very lowkappa number pulps can be produced (e.g. on the order of 2) prior tobleaching, and the liquids produced can be tailored to the particulardesired situation in which they are used in the pulp mill, with clearlyadvantageous results. The invention thus allows the production ofstronger pulp, having a lower kappa number, and/or higher yield,typically with less capital investment.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

I claim:
 1. A method of recovering chemicals during the production ofcellulose pulp using sulfur containing chemicals, comprising the stepsof:(a) providing a first fluid waste stream of sulfur containingchemicals from the production of cellulose pulp, having a first sulfurcontent; (b) providing a second fluid waste stream of sulfur-containingchemicals from the production of cellulose pulp having a second sulfurcontent, greater than the first sulfur content; the first and secondfluid waste streams being formed by heating a waste liquor to drive offsulfur-containing gases, recovering the sulfur from thesulfur-containing gases, and splitting the heated waste liquor into thefirst and second different waste streams; then (c) combusting the firstand second fluid waste streams separately to produce first and secondmelts, the second melt having the second sulfur content, and the firstmelt the first sulfur content; (d) prior to, or coincident with, step(c) adding the recovered sulfur only to the second of the split streams;(e) dissolving the first and second melts to produce first and secondliquors, the first liquor having the first sulfur content, and thesecond sulfur having the second sulfur content; and wherein step (c) ispractices by providing a soda recovery boiler having at least twodifferent melt sections, and wherein the first waste stream is led toone melt section, and the second waste stream is led to another meltsection.
 2. A method as recited in claim 1 comprising the further step(f) of adding other sulfur-containing constituents from the productionof cellulose pulp to the second split stream prior to or coincident withthe practice of step (c) for that stream.
 3. A method as recited inclaim 2 wherein step (f) is practices by adding one or more of thefollowing constituents to the second split stream:fly ash from a sodarecovery boiler; waste acid from a tall-oil plant; waste acid from achlorine dioxide plant; and sulfurous discharge gas from a pulpdigester.
 4. A method as recited in claim 1 wherein steps (a)-(e) arepracticed to produce a first liquor having a sulfidity of about 30% orless, and a second liquor having a sulfidity of about 50-90%.
 5. Amethod as recited in claim 4 comprising the further step of treating thefirst liquor with oxygen to increase the hydroxide ion concentrationthereof, and comprising the further step of, without interveningcausticization, treating the cellulose pulp in later stages of cook withthe hydroxide-ion concentration enhanced first liquor.
 6. A method asrecited in claim 4 comprising the further step of, without interveningcauticization, treating the cellulose pulp in the initial stages of cookwith the second liquor.
 7. A method as recited in claim 1 comprising thefurther step of causticizing one or both of the first and secondliquors.
 8. A method as recited in claim 7 wherein the first liquor iscausticized to produce white liquor, and comprising the further step ofoxidizing the white liquor so produced.
 9. A method as recited in claim8 wherein the second liquor is not cauticized, but is added directly tothe cellulose pulping without causticization.
 10. A method as recited inclaim 1 comprising the further step of oxidizing at least one of thefirst and second melts prior to the step (e).
 11. A method receivingchemicals during the production of cellulose pulp usingsulfur-containing chemicals, comprising the steps of:(a) providing afirst fluid waste stream of sulfur containing chemicals from theproduction of cellulose pulp, having a first sulfur content; (b)providing a second fluid waste stream of sulfur containing chemicalsfrom the production of cellulose pulp having a second sulfur content,greater than the first sulfur content; (c) combusting the first andsecond fluid waste streams separately to produce first and second melts,the second melt having the second sulfur content, and the first melt thefirst sulfur content; and (d) dissolving the first and second melts toproduce first and second liquors, the first liquor having the firstsulfur content, and the second sulfur having the second sulfur content;and wherein step (c) is practiced by providing a soda recovery boilerhaving at least two different melt sections, and wherein the first wastestream is led to one melt section, and the second waste stream is led toanother melt section.
 12. A method as recited in claim 11 wherein thefirst and second fluid waste streams are formed by heating a wasteliquor to drive off sulfur-containing gases, recovering the sulfur fromthe sulfur-containing gases, splitting he heated waste liquor into thefirst and second different waste streams prior to step (c); and priorto, or coincident with, step (c) adding the recovered sulfur to only thesecond of the split streams.
 13. A method as recited in claim 11comprising the further step of combining the waste gas streams from thedifferent melt sections of the recovery boiler.
 14. A method as recitedin claim 13 wherein the melt section producing the first melt isvertically about the melt section producing the second melt, off gasescontaining sodium being discharged from the section producing the firstmelt and reacting with off gases containing sulfur from the melt sectionproducing the second melt to enhance removal of sulfur from the offgases, and thereby lower the total discharge of sulfur from the recoveryboiler off gases.
 15. A method as recited in claim 11 comprising thefurther step of maintaining oxidizing conditions in the melt section towhich the first waste stream is led so as to produce oxidized meltcontaining Na₂ SO₄ as the first melt, and maintaining reducingconditions in the melt section to which the second waste stream is led.16. A method recovering chemical during the production of cellulose pulpusing sulfur-containing chemicals, comprising the steps of:(a) providinga first fluid waste stream of sulfur-containing chemicals from theproduction of cellulose pulp, having a first sulfur content; (b)providing a second fluid waste stream of sulfur-containing chemicalsfrom the production of cellulose pulp having a second sulfur content,greater than the first sulfur content; (c) combusting the first andsecond fluid waste streams separately to produce first and second melts,the second melt having the second sulfur content, and the first melt thefirst sulfur content; (d) dissolving the first and second melts toproduce first and second liquors, the first liquor having the firstsulfur content, and the second sulfur having the second sulfur content;steps (a)-(d) being practiced to produce a first liquor having asulfidity of about 30% or less, and a second liquor having a sulfidityof about 50-90%; (e) treating the first liquor with oxygen to increasethe hydroxide-ion concentration thereof; (f) without interveningcasuticization, treating the cellulose pulp in later stages of cook withthe hydroxide-ion concentration enhanced first liquor; and wherein step(c) is practiced by providing a soda recovery boiler having at least twodifferent melt sections, and wherein the first waste stream is led toone melt section, and the second waste stream is led to another meltsection.
 17. A method recovering chemicals during the production ofcellulose pulp using sulfur containing chemicals, comprising the stepsof:(a) providing a first fluid waste stream of sulfur containingchemicals from the production of cellulose pulp, having a first sulfurcontent; (b) providing a second fluid waste stream of sulfur-containingchemicals from the production of cellulose pulp having a second sulfurcontent, greater than the first sulfur content; (c) combusting the firstand second fluid waste streams separately to produce first and secondmelts, the second melt having the second sulfur content, and the firstmelt the first sulfur content; (d) dissolving the first and second meltsto produce first and second liquors, the first liquor having the firstsulfur content, and the second sulfur having the second sulfur content;(e) causticizing only the first liquor, to produce white liquor, andoxidizing the white liquor so produced; (f) adding the second liquordirectly to the cellulose pulp without causticization of the second; andwherein step (c) is practiced by providing a soda recovery boiler havingat least two different melt sections, and wherein the first waste streamis led to one melt section, and the second waste stream is led toanother melt section.